In a vehicular communication system, moving vehicles and roadside infrastructures are used as communicating nodes, and the vehicles and roadside infrastructures use wireless communication techniques to form a mobile network. The vehicles in the vehicular communication system which are neighboring or within a communication range (for example, 100 meters to 300 meters) connect to each other and exchange information such as safety warnings and traffic information. By transmitting movement notification between vehicles, the driver of a rear vehicle can be informed about the traffic status of the road ahead and have sufficient time to take action. For example, the driver of the rear vehicle is aware that slow traffic such as an accident or traffic jam occurs ahead and s/he may avoid driving through that area.
For the sake of illustration, a front vehicle is defined as vehicle A, and a rear vehicle is defined as vehicle B. In addition, how does a communication device of vehicle A (that is, communication device A) transmit movement notification related to vehicle A to a communication device of vehicle B (that is, communication device B) is illustrated as an example. It is noteworthy that, in practical application, communication device B may receive movement notifications from communication device of a number of vehicles, not only from communication device A.
In order to inform traffic condition to communication device B in advance, common techniques dynamically adjust the transmission rate (or transmission frequency) of the movement notification. That is, communication device A generates and transmits the movement notification to other communication devices according to measured movement of vehicle A. For example, when vehicle A is located at places where its movement may dramatically change (for example, at an intersection or a curve route), or when driver of vehicle A emerges brake, movement (such as velocity) of vehicle A is significantly decreased. In such case, communication device A transmits the movement notifications related to vehicle A with a higher transmission rate. However, communication devices placed at vehicles around vehicle A also send movement notification to vehicle B more frequent.
In the vehicular communication system, application notifications are broadcast to inform status of vehicles and events. These application notifications may include road hazard signaling (hereinafter, RHS), intersection collision risk warning (hereinafter, ICRW), signal violation warning (hereinafter, SVW), transit signal priority (hereinafter, TSP), pre-crash warning (hereinafter, PCW) and so forth. However, when these application notifications are generated and transmitted together, serious channel congestion may happen and important application notifications (especially for security ones) cannot be received in a real time manner. Therefore, in common techniques, the transmission rate of movement notifications is lowered in response to quality of communication bandwidth. Consequently, communication device A generates and transmits less movement notifications to communication device B.
For example, in a general case, communication device A is assumed to generate and transmit the movement notifications related to vehicle A in a transmission rate of 10 Hz. When an emergency event happens, area surrounding vehicle A becomes congest, and communication device A transmits the movement notifications with higher transmission rate (for example, 20 Hz). However, communication device placed at vehicles which are neighboring vehicle A also transmit their corresponding movement notifications with higher rate. As a result, the communication bandwidth becomes congest. In order to sooth the congestion of communication bandwidth, communication device A may instead lower down the transmission rate of the movement notification from 10 Hz to 4 Hz.
In a case that the velocity of vehicle B is assumed to be 100 kilometer/hour (that is, 27.78 meter/second), a transmission interval of movement notification will extend from 100 millisecond to 250 millisecond if the transmission rate of communication device A is decreased from 10 Hz to 4 Hz. Consequentially, communication device B receives the movement notification related to vehicle A after a delay duration, and vehicle B continuously moves forward during the delay duration. That is to say, position of vehicle B becomes closer to vehicle A.
In Table 1, the distance that vehicle B moves during the delay duration when transmission rate is 10 Hz and 4 Hz are compared. The columns in Table 1 respectively represent when the transmission rate of movement notification is 10 Hz (that is, receiving cycle is 100 ms), 4 Hz (that is, receiving cycle is 250 ms), and the distance difference between the transmission rate of 10 Hz and the transmission rate of 4 Hz.
The rows in Table 1 are respectively illustrated in a top-down sequence. The first row represents that the first movement notification sent by communication device A is successfully received by communication device B. The second row represents that communication device B misses the first movement notification and successfully receives the second movement notification. The third row represents that communication device B misses the first and the second movement notifications but receives the third movement notification.
TABLE 1transmissiontransmissionrate =rate =4 Hz10 Hztransmissiontransmissiondistancemoving distanceinterval =interval =differenceof vehicle B250 millisecond100 millisecond(meter)receiving first27.78 m/s * 0.2527.78 m/s * 0.16.94 − 2.778 =movements * 1 = 6.94s * 1 = 2.7784.162 metersnotificationmetersmetersreceiving second27.78 m/s * 0.2527.78 m/s * 0.113.88 − 5.556 =movements * 2 = 13.88s * 2 = 5.5568.324 metersnotificationmetersmetersreceiving third27.78 m/s * 0.2527.78 m/s * 0.120.83 − 8.334 =movement2 s * 3 = 0.83s * 3 = 8.33412.496 metersnotificationmetersmeters
According to the first row in Table 1, if the transmission rate of movement notification is changed from 10 Hz to 4 Hz and communication device B successfully receives the first movement notification, vehicle B will move 4.162 meters more and become close to vehicle A. According to the second row in Table 1, if the transmission rate of movement notification is changed from 10 Hz to 4 Hz and communication device B receives the second movement notification, vehicle B will move 8.324 meters more and become closer to vehicle A. According to the third row in Table 1, if the transmission rate of movement notification is changed from 10 Hz to 4 Hz and communication device B receives the third movement notification, vehicle B will move 12.496 meters more and become much more close to vehicle A.
When movement of vehicle A varies rapidly, vehicle B needs to acquire status of surrounding environment of vehicle A more often. However, increasing transmission rate of movement notification may result in the side effects that channel load becomes heavy and the movements notifications are congest. Consequentially, vehicle B may move forward and become closer to vehicle A. According to Table 1, when the transmission rate of movement notification is decreased, vehicle B is notified later and negative impacts on the vehicle status system may be caused.
As described above, the actual receiving rate of the movement notification may be decreased if the channel becomes congest. The channel congestion can be caused by that vehicles increasingly transmits movement notifications due to the easily changed movement variation. When traffic congestion happens, movement status of vehicle may not be reflected in a real time manner and the effect of the vehicular communication system is affected.